Modular visitor in scala

modularvisitor.scala

package modularexp

import scala.language.higherKinds

/**
  * Created by mleduc on 28/06/17.
  */
trait Exp[A]

trait Sum[A] extends Exp[A] {
  val lhs: Exp[A] with A
  val rhs: Exp[A] with A
}

trait Const[A] extends Exp[A] {
  val value: Int
}

trait VisitorSum[A] {
  def visitSum(e: Sum[Acceptor[A]]): A
}

trait VisitorConst[A] {
  def visitConst(e: Const[Acceptor[A]]): A
}

trait Acceptor[A] {
  def accept(): A
}

class PrettyPrintVisitor extends VisitorConst[String] with VisitorSum[String] {
  override def visitConst(e: Const[Acceptor[String]]): String = e.value.toString

  override def visitSum(e: Sum[Acceptor[String]]): String = s"(${e.lhs.accept()} + ${e.rhs.accept()})"
}


trait CalculateVisitorConst extends VisitorConst[Int] {
  override def visitConst(e: Const[Acceptor[Int]]): Int = e.value
}

trait CalculateVisitorSum extends VisitorSum[Int] {
  override def visitSum(e: Sum[Acceptor[Int]]): Int = e.lhs.accept() + e.rhs.accept()
}

object Test extends App {
  val ppv = new PrettyPrintVisitor

  def program[A](ppv: VisitorSum[A] with VisitorConst[A]): Sum[Acceptor[A]] with Acceptor[A] = {
    val buildSum = initBuildSum(ppv)
    val buildConst = initBuildConst(ppv)

    val lhs = buildConst(1)
    val rhs = buildConst(2)

    val program = buildSum(lhs, rhs)
    program

  }

  def initBuildSum[A](ppv: VisitorSum[A]): (Exp[Acceptor[A]] with Acceptor[A], Exp[Acceptor[A]] with Acceptor[A]) => Sum[Acceptor[A]] with Acceptor[A] = (lhs_, rhs_) => {
    new Sum[Acceptor[A]] with Acceptor[A] {
      override val lhs: Exp[Acceptor[A]] with Acceptor[A] = lhs_
      override val rhs: Exp[Acceptor[A]] with Acceptor[A] = rhs_

      override def accept() = ppv.visitSum(new Sum[Acceptor[A]] {
        override val lhs: Exp[Acceptor[A]] with Acceptor[A] = lhs_
        override val rhs: Exp[Acceptor[A]] with Acceptor[A] = rhs_
      })
    }
  }

  def initBuildConst[A](ppv: VisitorConst[A]): (Int) => Const[Acceptor[A]] with Acceptor[A] = (nbr) => {
    new Const[Acceptor[A]] with Acceptor[A] {
      override val value: Int = nbr

      override def accept(): A = ppv.visitConst(new Const[Acceptor[A]] {
        override val value: Int = nbr
      })
    }
  }

  println(program(ppv).accept())
  println(program(new CalculateVisitorConst with CalculateVisitorSum {}).accept())
}